The present invention relates to the field of lined explosive charges for perforating targets. More particularly, the present invention relates to an apparatus for generating a perforating jet which can produce a large hole in a target.
The invention is particularly useful in the field of downhole well perforation. Well casing is typically installed in boreholes drilled into subsurface geologic formations. The well casing prevents uncontrolled migration of subsurface fluids between different well zones and provides a conduit for production tubing in the well. The well casing also facilitates the running and installation of production tools in the well.
To produce reservoir fluids such as hydrocarbons from a subsurface geologic formation, the well casing is perforated by high velocity jets from perforating gun shaped charges. A firing head in the perforating gun detonates a primary explosive and ignites a booster charge connected to a primer or detonator cord. The detonator cord transmits a detonation wave to each shaped charge.
In a conventional shaped charge, booster charges within each shaped charge activate explosive material which collapse a shaped liner about a cavity. The collapsing liner generates a high velocity jet for penetrating the well casing and the surrounding geologic formations. The jet properties depend on the charge shape, released energy, and the liner mass and composition. Such jets perforate the well casing and establish a flow path for the reservoir fluids from the subsurface geologic formation to the interior of the well casing. Such flow path can also permit solid particles and chemicals to be pumped from the casing interior into the geologic formation during gravel packing operations.
Conventional shaped charges generate relatively small perforations in a well casing. In certain well completion operations such as gravel packing, large diameter well perforations are desirable to facilitate the rapid placement of the solid particles. Conventional perforations are formed with high velocity shaped charge jets or bullets, however the entry size of the resulting perforations is only slightly larger than the perforator diameter.
A need exists for an apparatus that can create large diameter perforations in well casing and other selected targets. Such an apparatus must remove a large target surface area before the energy of the perforating jet is expended. In addition, the apparatus should be capable of generating large perforations without damaging other downhole well components.